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How to Grow Annual Flowers

Customer photo by John R. of Salem, Mass.: “I put two windowboxes together to form a right angle in front of my boxwood bushes. I started all of the Limoncello petunias from seed using the Gardener's Supply seedstarter kit.”

Until a few years ago, if you wanted to grow annual flowers, your choices were limited to geraniums, impatiens, marigolds and red salvia. But today, most greenhouses offer an eyecatching array of choices. Whether you're planting a windowbox, lining your sidewalk, or spicing up your perennial garden, here's how to make the most of what's available.

Strictly speaking, an annual plant is one that completes its growing cycle (grows from seed, flowers and produces seed) in the course of a single growing season. In other words, annuals pack a lot of living into a short span of time.

But beyond this simple definition, there is an even wider range of plants that we treat as annuals. Some, such as impatiens, heliotrope and tuberous begonias, are actually tender or "half-hardy" perennials that can't survive even a light frost. On the other hand, some annuals, such as pansies and ornamental cabbage, are extremely cold-hardy and can withstand freezing temperatures quite well.

One of the best things about annuals is their incredible diversity and versatility. Using them allows you to compose really exciting combinations of color, form and texture that will last all season long. Colors range from bright midsummer favorites such as zinnias and Mexican sunflowers, to the subtler pastel shades of stock or lavatera.

You can also select annuals for your garden based on characteristics other than flower color. There are annuals that are tall, medium, short or climbing; ones that prefer either full sun or partial shade; and those with special virtues, such as delightful fragrance (stock, mignonette, nicotiana) or attractive foliage (caladium, coleus, dusty miller).

Perennials Grown as Annuals

A tender perennial is one that won't survive the winter in your climate. Many gardeners simply treat these plants as annuals, enjoying them for one season and letting them die in the fall. Other people move plants inside at the onset of cold weather: treating them as houseplants over the winter; taking cuttings and starting new plants; or simply digging up and storing part of the plant (usually the roots or bulblike structures) indoors for replanting the following year.

Examples of perennials that are commonly grown as annuals include the more tender flowering sages (Salvia coccinea, S. patens, S. splendens, etc.), verbenas, and hyssop (Agastache spp.). Geraniums and scented geraniums (Pelargonium spp.) can be grown outdoors either in beds or pots during the summer, then brought indoors at the end of the season: to bloom in pots, to store in darkness for replanting, or to use as cuttings for new plants. Petunias, coleus, and sweet-alyssum (Lobularia maritima) are other plants that can be overwintered in pots and replanted the following year.

If you have a sunspace or attached greenhouse that receives plenty of winter sunlight and doesn't get too cold at night, you might try growing some of the interesting "annuals" that in their native habitats are actually perennial shrubs and trees. For example, if given year-round protection, fuschias grow rapidly, reach anywhere from 18 inches to 12 feet or more, and produce their beautiful pendulous blossoms in shades of red, purple, and white nearly all winter long. Brugmansia, or angel's-trumpet, can grow to 15 feet tall in greenhouse cultivation. Its trumpet-shaped flowers are fragrant and beautiful, but don't grow it in the house if you have small children; the plants are extremely poisonous.

Flowering plants that grow from tender bulbs, such as dahlias, gladioluses and cannas, are often planted as annual flowers in cutting gardens or mixed ornamental borders. They, too, are tender perennials, and most varieties won't survive the winter outdoors in most of North America. However, it's easy to dig up these bulbs at the end of the growing season and store them indoors for replanting the following year.

Starting Annuals from Seed

Seeing all the annuals offered for sale at nurseries and garden centers in the spring, you might wonder who would go to all the trouble of starting their own annuals from seed. There is an economic advantage, of course. A $2 packet of seeds might grow four large flats of alyssum plants, which would be a savings of roughly $70 over buying the plants from a nursery.

An equally compelling reason for starting your own plants from seed is that even the best garden centers don't carry the full selection of worthy garden annuals. Most sell the popular bedding plants, but it can be hard to find old-fashioned annuals, such as love-lies-bleeding (Amaranthus caudatus), four o’clocks (Mirabilis jalapa), or lavatera (Lavatera trimestris)?

Part of the reason for this absence stems from economics (supply and demand) and part is due to the nature of the plants themselves. Garden centers like to sell six-packs of plants that are already in bloom; that way customers know what they're getting, and know that the plants will probably continue blooming after they get them home and in the ground. Instant gratification.

Yet many fine annuals won't start blooming in nursery six-packs. They're either too tall, don't like to be transplanted, or just won't flower until they have been in the ground for a couple of weeks. If you want to experiment with the whole palette of annuals, eventually you will want to grow some of your own plants from seed.

Many annuals are easy to seed directly into garden soil. Others are best started indoors under lights in late winter or early spring. Consult seed catalogs, seed packets or the book From Seed to Bloom by Eileen Powell for information on specific plants. Generally speaking, annuals fall into three main categories, which determine when and where you should sow their seeds. For all categories, a good rule of thumb is to plant seeds at a depth of two or three times their diameter.

Some annuals are so good at fulfilling their mission in life—flowering and setting seeds—that they will self-sow readily under the right conditions and produce brand-new plants the following year. Common annuals that can self-sow vigorously include ageratum, petunia, foxglove, annual larkspur, forget-me-not, calendula and wild or striped mallow (Malva sylvestris).

Hardy annuals: Can be direct-sown in the garden as early in the spring as the soil can be worked. For an earlier start, sow them indoors in flats eight to ten weeks before the last spring frost date, and transplant them to the garden about a month later, after hardening them off.

Some hardy annuals can also be direct-seeded in the fall, and these plants will flower much earlier than plants seeded in the spring. When fall seeding, plant the seeds a bit deeper than you would in the spring, and spread some mulch over the seedbed after the ground has frozen.

Half-hardy annuals: These can be direct-sown outdoors after the threat of hard frost (temperatures below 25°F) is past. Indoors, start seeds in flats six to eight weeks before the last spring frost date, and harden off the plants before transplanting them to the garden. Once they have hardened off, half-hardy annuals can withstand a light frost.

Tender annuals: Seed can be sown directly in the garden only after all danger of frost is past. For an earlier start, sow seed indoors four to six weeks before the last spring frost date for your area. Examples include marigolds, morning glories, zinnias, sunflowers and tithonia (Mexican sunflower), cosmos, amaranth, ageratum, celosia and gomphrena (globe amaranth).

Propagating from Cuttings

Late summer or early fall is an ideal time to take cuttings from annuals such as geraniums, coleus and impatiens, potting them up for winter bloom indoors or to hold them over for the following spring. The following steps will help ensure success.

Clip off any flowers or flower buds on the plant. This is done to focus the plant’s energy into developing new roots on the stem cutting.

Select healthy stem cuttings (preferably healthy growing tips or side shoots) that are 2 to 6 inches long. Strip off any bottom leaves where the stem will be inserted in the rooting medium (either potting soil or water). Dip the cut end of the stem into a rooting hormone powder to encourage rapid root growth.

Insert the cutting in potting soil (not a soilless seedstarting mix) and water the container. Cover the flat or pot with a clear plastic bag to create a moist, humid atmosphere. Don’t let the plant leaves touch the side of the bag because this will cause rot.

New roots should develop in one to three weeks. To test, gently tug on the cutting. Pot up the new plants in 4- to 6-inch containers and keep out of direct sunlight for three days. After this time, place the plants in a sunny location.

An alternative method of rooting plants, such as begonias, coleus or geraniums, is to place the stem cuttings in a glass of water to develop roots. Change water every few days until plants develop roots, and add a little soil to the jar after new roots appear. Plant rooted cuttings in 4- to 6-inch containers filled with potting soil.

Once the new plant is well established, pinch off the stem tip to encourage fuller growth and more abundant flowers.

Growing and Care of Annuals

Soil preparation and planting: In general, annuals prefer well-drained soil with a pH between 6.3 and 6.7. Digging in a good quantity of peat moss or compost will help to build up the soil's organic matter and allow the plants' roots to spread quickly and get off to a good start.

Set out young plants at the recommended spacing, to prevent them from crowding each other once they have grown and matured. If you've purchased plants in flats from a garden center, the plants will likely be somewhat potbound when you remove them from their cells or containers. Before placing them in the planting hole, gently break apart the root mass; this encourages roots to spread quickly into the surrounding soil. Fertilize at planting time with an organic or slow-release fertilizer.

You need to pay attention to whether a particular plant is hardy, half-hardy or tender before deciding when to transplant it. Some nurseries sell plants with labels that identify hardiness; when in doubt, put your annuals out after all danger of frost has passed. Another crucial factor, of course, is whether a particular plant prefers sun or partial shade.

If you need to hold plants in flats for more than a couple of days at home, be sure to water them and keep them in a partly shady, protected spot, such as a porch or under a tree. Don’t leave them in a garage or other structure where you store cars or gasoline-powered machines; ethylene gas can cause flower damage and leaf drop. The best advice is to transplant annuals to the garden as soon as possible after bringing them home.

Care of annuals: Once they start blooming, most annuals will flower all season long, until cold temperatures or frost put an end to their display. However, to keep them flowering and looking good, you will need to perform some simple but easy maintenance.

Deadheading is the most important task, and it involves pinching off old flowers just as they begin to fade. The reason for doing this is simple. Annuals live to flower quickly, produce seed and die. So long as you keep deadheading blossoms, the plants will continue to produce flowers; once you stop, the plants will reduce or stop flowering, and put their energy into maturing seeds. Pinching off spent blooms is quick and easy, and it ensures season-long bloom.

If you've fertilized at planting time as recommended with an granular, organic, slow-release fertilizer, you shouldn’t have to fertilize annuals again during the season. With annuals, the flowers are the thing, and overfertilizing can lead to lush foliage growth, which is really beside the point. The primary exception is container-grown plants, which usually need to be fertilized with water-soluble fertilizer every couple of weeks to maintain a colorful show.

Annuals have shallow root systems and so require a regular supply of water. Avoid overhead watering if possible, which can stain some types of flowers (such as petunias), and make them look unattractive. It also can contribute to a buildup of botrytis fungus, which can affect plants such as zinnias, geraniums, and marigolds. For best results, use a soaker hose or a drip irrigation system, or direct your watering can right at soil level.

Designing With Annuals

When shopping for annuals, it helps to think in terms of color and form.

Because they only stay in the garden for one season, annuals offer maximum flexibility. If you don't like the effect you've created one year, you can simply chalk it up to experience and try again next year, without having to move plants around as you would with perennial plants.

The most popular and widely grown annuals are used as bedding plants—combinations of brightly colored flowers and foliage plants in a bed that is accessible from all sides for visibility and ease of maintenance. Such formal plantings can be especially effective if you plant a solid block of plants of the same variety and color. Separated by neat strips of lawn, such single-color plantings lend a nice formal effect to the garden.

An even more impressive sight is a massed single-color planting divided down the center by a band composed of a flower that has a different, but complementary, color or growing habit. For instance, a dark, vivid color, such as the bluish purple flowers of border lobelia (Lobelia erinus), might combine well with the white flower mounds of sweet alyssum (Lobularia maritima). Both plants are tender perennials grown as annuals, and both are similar in growing habit: low, mounding and normally used for edging beds.

Annuals work equally well in less formal designs, and they fill an important role by giving you a brilliant palette of color with which to work. They can be inserted into open spots between and around perennials and flowering shrubs. Adding annuals to a perennial border can bolster the effect of the whole, ensuring a continuity of color and interest even when the perennials are not in bloom. They are great for creating rhythmic splashes of color, for linking different parts of the garden together, and for helping to carry a particular color theme through the garden during the entire growing season.

Some of the best annuals for mixed border plantings include tall species such as nicotiana (flowering tobacco), cleome (spider flower), lavatera, Shirley poppies, foxglove, matricaria or the vibrant orange Mexican sunflower (Tithonia rotundifolia). For mid- and late-season color at the front of the border, use annuals such as impatiens, matricaria, Salvia horminium and sweet alyssum.

Sweet alyssum is one of the most popular annuals.

With all their different heights, colors and forms, it's entirely possible to plant a spectacular border composed of annuals alone. Since most annuals flower at the same time, and over an extended season, you may want to choose varieties that will complement one another. You can strive to create a particular color scheme (pink, blue, and white; yellow, blue and orange), or simply go for a full-blown riot of color.

One way to create a more interesting and designerly effect is to include annuals that are grown for their attractive foliage, which can act as a foil for the bright blooms of other plants. The silver foliage of dusty miller (Senecio cineraria) is an old standby in the annual garden, but there are lots of other, lesser-known foliage plants as well, including cannas, which have tropical-looking, sometimes bronzed leaves; plectranthus, with its soft, silvery leaves; and perilla (Perilla frutescens), a beautiful herb whose dark purple, fringed leaves are particularly effective with white and pink-purple flowers.

Climbing annuals are another good choice, especially for cottage garden settings and containers (window boxes, hanging baskets, etc.). They have an old-fashioned, informal quality and will create a colorful living tapestry on fences, screens, trellises, or other supports. Plenty of people grow morning glories (Ipomoea purpurea and I. tricolor) and their close relatives, moonflower (I. alba) and cardinal climber (I. x multifida). But that scarcely scratches the surface of great climbers..

Containers and Overwintering

The perfect choice for growing in containers, annuals work well either alone or in combined plantings. Be imaginative when selecting containers, and if you have the room and the resources, don't just stick to the tried-and-true terra-cotta pots. Windowboxes are designed for annuals, especially ones that cascade over the sides. The same holds true for hanging containers, where trailing varieties, such as the old-fashioned nasturtium Empress of India', hang down and make a pretty display. For more information, read Planting a Winning Container.

Impatiens are particularly beautiful when planted in a container set in filtered shade. Most commonly you'll see them planted in the round, usually in a large container like a half whiskey barrel. To set your display apart, try experimenting with something a little different. An old soapstone sink would make an excellent bed for impatiens or other colorful annuals, as would a planter you've either bought or made from hypertufa (a kind of artificial stone). Brown fiber pots and planters are widely available, and will last for several seasons. They're also light (at least when empty) and easy to move.

When planting in containers, make sure that you allow for adequate drainage. If the pot or container doesn't have drainage holes, either insert a smaller container that does inside of it, or cover the bottom of the container with a layer of small stones or foam peanuts (a terrific way to recycle them).

Like other plants grown in containers, annuals require regular watering and fertilization throughout the growing season. In addition, you'll also have to deadhead spent blossoms. During hot, dry weather, your plants may need watering once or even twice a day. Certain plants, such as moss rose (Portulaca grandiflora) and calliopsis (Coreopsis tinctoria), prefer somewhat dry soil and hot, sunny weather.

Tender annuals should be protected from early frosts. A stretch of warm fall days often follows the first frosts, so it pays to cover your plants and prolong the season. Once cool weather becomes the norm and frost kills off your plants, remove them from their pots and clean the containers with soap and water or a dilute bleach solution, to get them ready for next year.

The end of the growing season doesn't necessarily mean the end of your annuals. Tender perennial plants already growing in containers can be cut back and brought indoors, while bedding plants such as petunias, impatiens, lantana and geraniums can be potted up and treated to a prolonged season of bloom inside.

Before bringing these plants indoors, check them for insect or disease problems and either treat them or discard them if you find any. Cut back the plants by 4 to 6 inches, and place the pots in a room that gets a lot of light. A greenhouse or sunspace is ideal, but so are sunny windows with a west or south exposure. After cutting back the plants, give them a dose of liquid plant food and they should soon start to develop new leaves and flowers.

Keep a close eye on any plants you bring indoors, at least for the first couple of weeks. The shock of being moved inside makes plants very vulnerable until they become acclimated to their new growing conditions. Pamper your plants as much as you can at this time to ease their transition to indoor life.

Some plants can be dug up in the fall, pruned way back, and stored in a cool, dark place until early spring. This includes most of the tender perennials, such as brugmansia, datura and geraniums. Water plants sparingly during this time.

Another strategy for overwintering plants is to take cuttings in the summer or early fall and start new plants from your old annuals. This technique works particularly well with coleus, plectranthus, and licorice plant (Helichrysum petiolare).